The present invention relates to a device for reading on a pixel basis images which are carried on an original document.
In an image recording and reproducing system of the type using a dot matrix printer, it is a common practice to optically scan an image-carrying surface of a document by a scanner to sample and read images on a pixel basis and, then, convert the resultant analog data to digital data, thereby producing digital image data. This process usually involves spatial filter processing which is an implementation for enhancing the quality of image recording. Specifically, the spatial filter processing is adapted to accentuate the edges of images to thereby compensate for blurring of images, or to smooth images.
The spatial filter processing uses a weighted N.times.N matrix and extracts each digital image data from a particular image area which has the same size as the matrix, i.e. N.times.N, and has at its center a particular pixel to be processed (hereinafter referred to as an observed pixel). The digital image data and the weights of the filter are subjected to predetermined arithmetic operation to compensate the value of the digital image data of the observed pixel. A dilemmatic situation with such processing is that while the matrix size should desirably be increased to apply the processing to a wider pixel area to enhance the image quality, a larger matrix size results in the need for a correspondingly greater capacity of a buffer memory which serves to temporarily store each digital image data associated with a particular pixel area, as well as for a longer period of time for the operation. In practice, therefore, use is made of a matrix whose size is on the order of 3.times.3 sacrificing the image quality to a certain extent.
However, assuming that the document image sampling pitch is on the order of 62 .mu.m (16 pixels/mm), for example, the spatial filtering with a 3.times.3 matrix is effective only for those image frequencies which are higher than 5 pixels/mm. To process image frequencies on the order of 1 to 3 pixels/mm which cover the peak of modulation transfer function (MTF) against eyesight, a spatial filter having a greater size such as 5.times.5, 7.times.7 or 9.times.9 is required.